Moparts
vacuum advance
Posted By: zrxkawboy
vacuum advance - 05/03/12 11:15 PM
The thread on the overheating 67 Belvedere got me thinking, but I didn't want to hijack the thread. This is something I've wondered about from time to time: why do some people not run vacuum advance on street-driven cars? I'm sure no expert, but it seems to me that it would always be beneficial. Any thoughts?
Posted By: GTX MATT
Re: vacuum advance - 05/03/12 11:20 PM
Well for one, drag cars don't us mechanical advance so some people plug it thinking that it will give them a performance increase.
More likely they re-curved their distributor, or set the timing to get more total advance, or over advanced their timing. Either way some effort has been made to tune the car with a more aggressive timing curve for more power. Then they run into issues at part throttle (usually in high gear on hills) with detonation, so plugging the vacuum advance cures this. I don't like plugging the vacuum advance, it makes a lot of cars (particularly automatics) feel very sluggish under normal driving. But to each their own. Sometimes it may be necessary but I think it often gets plugged when it shouldn't be.
Posted By: CUDAJAS
Re: vacuum advance - 05/04/12 01:08 AM
I have run my smblk cuda with and with out.
My experinace has been varied. With the vac advance hooked up to ported vac (above the throtle blades) the car bucks and jumps under slow crusing speeds and decell. Tried to tune it out but no luck.
With the vac advance hooked to constant vac, below thotle bades, the idle smoothed out (increase timing), but under part throtle cruise, highway speed, the car ran incredbibly lean / stinky and 10 - 15 deg hotter than normal. Really not good! Had to stop 45 min into a road trip to disconnect the vac advance.
With the vac adacne disconnected, the car runs well, idelas well (choppy but that is the idea).
Posted By: cogen80
Re: vacuum advance - 05/04/12 01:14 AM
Quote:
The thread on the overheating 67 Belvedere got me thinking, but I didn't want to hijack the thread. This is something I've wondered about from time to time: why do some people not run vacuum advance on street-driven cars? I'm sure no expert, but it seems to me that it would always be beneficial. Any thoughts?
because guy are scared to mess with it, don't understand it, or just old and stubborn.
Posted By: Mr.Yuck
Re: vacuum advance - 05/04/12 01:28 AM
define your "street driven car" it's not really needed for anything but economy. If you have even a mild 440 Lock the sucker in at 38* and be on your way. The MPG different will be minimal at best. However if your junk is advancing up to 50-55* you could hurt something.
Posted By: rbstroker
Re: vacuum advance - 05/04/12 01:44 PM
In my efforts to make my stroker more streetable, I had a vacuum advance distributor set up by 4 seconds flat. Replaced my non-vacuum tach drive distributor. After swapping and some carb tweaking, I have to say that it is better.
Posted By: lewtot184
Re: vacuum advance - 05/04/12 02:31 PM
Quote:
In my efforts to make my stroker more streetable, I had a vacuum advance distributor set up by 4 seconds flat. Replaced my non-vacuum tach drive distributor. After swapping and some carb tweaking, I have to say that it is better.
i got away from the full centrifugal only distributor for street driving many years ago. a vacuum advance distributor is better for street use. lets the engine burn cleaner.
Posted By: can.al
Re: vacuum advance - 05/04/12 02:56 PM
..very important feature for street driven cars and your car will run o.k. without it but you're leaving a lot on the table.
google "timing and vacuum advance 101".. and you will be without excuse
Posted By: 383man
Re: vacuum advance - 05/04/12 03:50 PM
I guess for me its because I am an old bracket racer from the 70's and under full throttle like drag racing the vacum advance does not work. And I like very fast mech advance as my dist has full advance by 2000 rpm. And it is a race unit without a vacum advance. Right now I have 38 total in very early and it runs great. Never tried a vacum advance but of course I would have to change dist to try it. Ron
Posted By: DARTH V8Я
Re: vacuum advance - 05/04/12 04:01 PM
Quote:
define your "street driven car" it's not really needed for anything but economy. If you have even a mild 440 Lock the sucker in at 38* and be on your way. The MPG different will be minimal at best. However if your junk is advancing up to 50-55* you could hurt something.
Locked out dizzy eh? 
Guess some people refuse to move ahead 40 years 
Posted By: Challenger 1
Re: vacuum advance - 05/04/12 04:08 PM
Quote:
Quote:
define your "street driven car" it's not really needed for anything but economy. If you have even a mild 440 Lock the sucker in at 38* and be on your way. The MPG different will be minimal at best. However if your junk is advancing up to 50-55* you could hurt something.
Locked out dizzy eh? Guess some people refuse to move ahead 40 years
Wow yuck, better read up before posting.
Posted By: lewtot184
Re: vacuum advance - 05/04/12 04:52 PM
Quote:
I guess for me its because I am an old bracket racer from the 70's and under full throttle like drag racing the vacum advance does not work. And I like very fast mech advance as my dist has full advance by 2000 rpm. And it is a race unit without a vacum advance. Right now I have 38 total in very early and it runs great. Never tried a vacum advance but of course I would have to change dist to try it. Ron
in my 65 coronet i use an aggressive centrifugal advance with the vacuum. at first i didn't think it would work but it seems to be a happy set-up. the distributor has a 9 degree plate, full centrifugal is all in at 2200rpm, and a 9 degree vacuum can. i'm sure the engine is burning cleaner with it, especially at cruise, and of course at full throttle the vacuum advance doesn't work.
Posted By: mikemee1331
Re: vacuum advance - 05/04/12 10:13 PM
i'm also lining up on the 'use it' side. i have played with it both ways and while i won't say my modified 440 6pack setup is a turd without it (how can such awesomeness ever be one
) there is a huge difference on the street in 'seat-of-the-pants feel and response.
Posted By: Mick70RR
Re: vacuum advance - 05/04/12 10:25 PM
I prefer to use vacuum advance for street driving but I limit it to 10/12 degrees. Mechanical is all in by 2500 rpm and vacuum is connected to the manifold. I've ran both ported vacuum and manifold vacuum and had good results with both on different engines. Manifold vacuum suits my current set-up, it idles a lot better with the additional advance so that's what I run. Also tried disconnecting the vacuum advance and my right foot is nearer the carpet when I'm cruising.
The bottom line: If you don't have at least 50-55 deg. (total) at lightest throttle / cruise, you're making the Arabs rich and cutting ring / cyl wall life in half!
Assuming you have a canister that's marked. say, 8 deg. on the arm (16 crankshaft degrees), you'll be close to dialed in as far as overall advance with the typ. 33-35 deg. (total of initial and mech). You vary the spring pressure on the back of the diaphragm to tune out any detonation.
This is not very different than virtually all muscle-era Mopar distributors were stock.
Rick
Quote:
The bottom line: If you don't have at least 50-55 deg. (total) at lightest throttle / cruise, you're making the Arabs rich and cutting ring / cyl wall life in half!
Assuming you have a canister that's marked. say, 8 deg. on the arm (16 crankshaft degrees), you'll be close to dialed in as far as overall advance with the typ. 33-35 deg. (total of initial and mech). You vary the spring pressure on the back of the diaphragm to tune out any detonation.
This is not very different than virtually all muscle-era Mopar distributors were stock.
Rick
Is there an upper end of too much vac advance? I think I saw 60* on my timing tape the other day 
I really need to get a mity-vac to test it properly. I don't remember how many degrees it has stamped on the arm.
Posted By: DaytonaTurbo
Re: vacuum advance - 05/04/12 11:51 PM
Quote:
Is there an upper end of too much vac advance? I think I saw 60* on my timing tape the other day I really need to get a mity-vac to test it properly. I don't remember how many degrees it has stamped on the arm.
If you're not getting any detonation while at cruise or a very light acceleration then I'd say you're fine.
Posted By: ahy
Re: vacuum advance - 05/04/12 11:57 PM
As I understand it, the goal is to light the fire early (advanced) so you get maximum cylinder pressure built up near TDC. The efficiency gain comes from getting the benefit of that pressure for the full power stroke plus higher peak pressure near TDC.
With a "fast burn" combustion chamber lighting the fire too early could get max pressure before TDC and really fight the piston going up. Plus it could cause detonation.
Slow burn chambers like the 906 can benefit from more advance than the fast burn.
On my (sort of) fast burn BB with closed chamber Ed heads I have it set at 50 degrees via EFI. If I could figure out how to do it I'd paste the advance table. Is this optimal? I don't know It would take quite some testing and/or experiments to figure out. I do know it works well. The engine runs well and will get 14 MPG @ 75 MPH or the same on a 2 lane @ 65 MPH with an occacional blast for passing. Not bad for a 496 with 243 @.050 cam.
Quote:
Quote:
Is there an upper end of too much vac advance? I think I saw 60* on my timing tape the other day I really need to get a mity-vac to test it properly. I don't remember how many degrees it has stamped on the arm.
If you're not getting any detonation while at cruise or a very light acceleration then I'd say you're fine.
Well, I'm not getting any that I can hear over general engine/road/wind/rattle noise
Posted By: DARTH V8Я
Re: vacuum advance - 05/05/12 12:15 AM
If you have a knock sensor that would be optimal. The computer will give as much advance as possible. Watching my Truck, it'll try and 'hold' 50 degrees at all throttle conditions (cept' for full throttle). I generally never see it go lower then 35 or so even with a very heavy load.
Just remember that any detonation you can hear is very bad.
If you really get "into" the tuning thing, you'd be amazed how much light-throttle advance you can run, and how lean you can go, too. The only potential problem is rotor phasing causing misfire. That's why DIS / COP ignition is so great.
Rick
Posted By: 360904
Re: vacuum advance - 05/06/12 01:16 AM
whats the lowest number you can get a vacuum advance in? my car only has between 7 and 8 of vacuum at idle.
Posted By: lewtot184
Re: vacuum advance - 05/06/12 01:37 AM
Quote:
whats the lowest number you can get a vacuum advance in? my car only has between 7 and 8 of vacuum at idle.
i've gotten them to start opening around 4 inches. they are adjustable.
Posted By: ahy
Re: vacuum advance - 05/06/12 01:41 AM
Under light cruise, higher RPM with small throttle opening increases the vacuum significantly. Your 7-8 will likley be in the teens at cruise speed. That's when you need the vacuum advance.
Posted By: 360904
Re: vacuum advance - 05/06/12 01:58 AM
i think the one on my distributor now is a 8.5 does that sound right? I will have to hook it up and see what happens.